Localisation of a mobile terminal such as User Equipment (UE) involves determining an estimate of the geographical location of the mobile terminal. Location estimation of a mobile terminal is becoming increasingly important in wireless applications since it enables location dependent applications to be implemented, for example, in location based services and context awareness applications. Such services and applications may include, for example, accessing local emergency services, or providing users with useful information based on their geographical position such as the location of the nearest service station or restaurant. Consequently, the integration of location capability is becoming a mandatory feature on some UEs.
Existing solutions for localising mobile terminals are based on the determination of the propagation delay of a given signal emitted by an antenna of a transmitter (e.g. a Base Station (BS)) to a given mobile terminal. The determined propagation delay allows the distance between the transmitter and the mobile terminal to be calculated and thus the position of the mobile terminal to be derived by, for example, using several transmitters and a traditional triangulation method known to the man skilled in the art.
One existing solution for determining the distance between a transmitter and a UE using the propagation delay tp is illustrated in FIG. 1A. This technique is sometimes referred to as the Time of Arrival (TOA) or One-Way (OW) ranging (i.e. distance estimation) method. A signal or electromagnetic wave is sent in the form of packets from a transmitter 11 to a mobile terminal 12. The transmitter 11 and the mobile terminal 12 are, in this case, synchronized in time as well as frequency. Assuming that the velocity of the signal (i.e. an electromagnetic wave) is equal to c and that the propagation delay of a given transmitted packet is tp, an estimation of the distance between the transmitter 11 and the mobile terminal 12 may be given by:d=tp/c  (1)tp=TRB−TTA  (2)
where TTA and TRB are, respectively, the departure time of the signal from the transmitter 11, and the arrival time of the transmitted packet at the mobile terminal 12. In this solution, a packet is sent only in the direction from the transmitter 11 to the mobile terminal 12.
In some cases, a transmitter and a mobile terminal may not have a common clock (i.e. may not be synchronized in time), because they are located in distant locations to one other. If there is a time offset Δt between the clock of the transmitter and the clock of the mobile terminal, an accurate determination of the propagation delay tp may be made as depicted in FIG. 1B by taking this time offset into account. FIG. 1B illustrates the principle of Two-Way Ranging (TWR) method, in which a transmitter 11 sends a first data packet to a mobile terminal 12 and the mobile terminal 12 sends a second packet to the transmitter 11. The departure time of the first packet from the transmitter and its arrival time at the terminal are respectively denoted TTA and TRB. The departure time of the second packet from the UE and its arrival time at transmitter are respectively denoted TTB and TRA. Using the time offset Δt and the propagation delay tp:TTA+tp=TRB+Δt TRA−Δt=TTB+tp  (3) & (4)and, finally:
                              d          =                                    t              p                        /            c                          ⁢                                  ⁢                              t            p                    =                                                    (                                                      T                    R                    A                                    -                                      T                    T                    A                                                  )                            -                              (                                                      T                    T                    B                                    -                                      T                    r                    B                                                  )                                      2                                              (        5        )            
The departure time of a packet from a transmitter can be provided as a time stamp included in the data field of the transmitted data packet. In the case of the receiver, however, the receiver of the mobile terminal should be able to accurately estimate the arrival time of the data packet. This may be done by detecting the first arrival path in the estimated impulse response of the receiver to the incoming signal.
In telecommunications systems such as 3 G and 3.9 G systems a wideband signal such as a spread spectrum (SS) signal with good auto-correlation characteristics is generally employed. The receiver can thereby estimate the impulse response with a SS signal embedded in a data packet transmitted from the transmitter. In such a case, the receiver can select a starting point of a correlation window in the preamble where the SS signal is embedded. The time occurrence of the first arrival path is used to indicate the arrival time of the packet. The first arrival path refers to the path with the shortest delay in the multi-path profile of the receiver response, and thus corresponds to the shortest propagation path between the transmitter and the receiver. The first arrival path detection method consists thus in detecting at the receiver the first impulsion of a given impulse response to a received SS signal as illustrated on FIG. 1C. The receiver should accurately estimate the time of the first arrival path in order to estimate the time of arrival of the packet from the transmitter. This estimation can then in turn give a measurement of the distance between the transmitter and the receiver. In FIG. 1C, Δt corresponds to the time difference between the starting point of correlation of arrival of the packet containing the SS signal and the actual arrival time of the SS signal represented by the time occurrence of the first impulsion of the receiver response. Using the time stamp indicating the departure time, and the time of the first arrival path, the propagation distance between the transmitter and the receiver may be determined, and used to estimate the co-ordinates of the receiver.
Existing solutions for estimating the propagation delay concern Single-In Single Out (SISO) architecture in which transmitter delimiting a cell is provided with only one antenna and only one antenna is provided on the receiver (i.e. on the UE). Consequently, in the SISO architecture, there is a single departure point and single arrival point for the signal between the transmitter and the receiver.
When the mobile terminal is located at the edge of the cell, due to the long propagation distance between the transmitter and the mobile terminal, the Signal to Noise Ratio (SNR) tends to be small. As a result, the noise in the estimated impulse response becomes more significant leading to inaccurate first path detection.
Consequently, when the SNR is small, in order to improve the SNR (in other words, to suppress the effect of noise), the estimation of impulse response is repeated many times by transmitting a number of packets from the transmitter to the receiver in order to determine an average or normal.
FIG. 1D illustrates the first arrival path detection in the case of a SISO. Assuming the arrival of the first path in the k-th estimation of the impulse response as Δtk, the average arrival time of the first path is given by:
                              Δ          ⁢                                          ⁢                      t            ^                          =                              ∑                          k              =              1                        K                    ⁢                                    1              K                        ⁢            Δ            ⁢                                                  ⁢                                          t                k                            .                                                          (        6        )            
A drawback of this procedure is that since the process requires K estimation processes of the impulse response, i.e. K transmissions of a data packet, it leads to an estimation delay. Furthermore, when the speed of the mobile terminal is high, the averaging operation leads to a large error in estimating the time position of the first arrival path. It is desirable, therefore, that the estimation of the impulse response should be as quick as possible in order to give accurate first arrival path detection.
Further errors in location estimation result from the lack of direct path between the transmitter and the mobile terminal, otherwise known as non-line of sight (NLOS). This can occur, for example, in an urban environment, in which an obstacle such as a building may be located in the path between the transmitter and the mobile terminal. A signal propagating from the transmitter to the mobile terminal may be reflected or scattered by such an obstacle resulting in the signal travelling a greater distance from transmitter to mobile terminal than the actual distance between the transmitter and the mobile terminal.